When developing subsea oil and gas wells there are stringent demands to the control and containment of the well during all aspects of the work, be it drilling, production or later intervention. The needs for control of well pressure have lead to requirements for safe barriers in the well and/or the Christmas tree, both during production and during intervention work.
In a horizontal type Christmas tree the production tubing hanger is located within the vertical bore of the tree and includes a vertical tubing hanger bore and a horizontal passage which is aligned with the horizontal production outlet in the tree. The production control valves are located in the horizontal outlet. The tubing hanger passage extends above the production outlet to provide access to the well, and this passage must be closed off during production.
In a conventional Christmas tree, the tubing hanger is set in the wellhead and the production control valves are located in the vertical passage of the tree. The control valves function as barriers during normal production, and the tubing hanger includes means for setting a plug in its vertical bore in case the well must be closed down.
In many countries, rules require at least two barriers between the well and the environment. For example, in a horizontal completion the two barriers are set with one in the tubing hanger above the production passage and the other in the tree bore.
In U.S. Pat. No. 6,050,339 there is shown a horizontal Christmas tree of a type well known in the art. A first plug is set to close tubing hanger passage above the production outlet. An internal tree cap is set in the vertical bore of the tree above the tubing hanger. The tree cap has an axial bore in which a second plug is set. To gain access to the well, both plugs must be removed.
In U.S. Pat. No. 5,575,336 there is shown another type of a horizontal Christmas tree. As above a first plug is set in the tubing hanger. A tree cap is set in the tree bore above the tubing hanger. A ball valve is located in the tree cap passage. To gain access to the well a tool is run in to open the valve. Thereafter the plug can be removed. This can be done in one operation, thus saving a run.
During the lifetime of the well various types of work may be carried out to enhance production or to measure conditions in the well. To gain access to a living well a pressure containment device, containing a number of valves, must be connected to the well before the barriers can be removed. The pressure containment device ensures control over the well during the work. Depending on the type of the work, either a lubricator stack or a riser is connected to the pressure containment device.
Various parameters dictate the size and complexity of the equipment used during intervention, the main concern being the pressure control valves. One of the principal parameters is well pressure. Higher pressure ratings demand larger equipment. Another parameter is the nominal size of the equipment, related to the tool to be employed during intervention.
In recent years larger type completions have become the norm, with tubing sizes up to 9″. The size of the intervention equipment has therefore also grown bigger, since the internal diameter of the pressure control valves and workover riser must be large enough to accommodate the correspondingly sized plug(s). At the same time, intervention tools have become smaller, down to perhaps 2″ or less. Therefore the size of the plug(s) limits the scaling down of the equipment. This means that smaller equipment cannot be employed even if only small size tools are needed for the work in the well. For example, a standard type completion has a 6″ tubing and therefore a 6″ plug. The equipment must therefore be dimensioned with a nominal 6″ bore even if the tool to be used is only 2″ and could be run in on cable or wireline.
The size and weight of the equipment also influence the choice of the type of vessel used for well intervention, especially governing the load handling capabilities of the vessel. As an example, a complex operation like pulling the tubing requires a full blow out preventer and drilling riser, resulting in the need for using a large drilling rig that can handle this load. At the other end of the scale, a simple sensor can be run in on slickline or cable requiring only a small boat.
Reducing the size of the equipment can therefore reduce costs dramatically. As an example, if the equipment can be reduced to 4″ nominal size, the weight can be reduced by more than 30% as compared with 6″ equipment. This again allows smaller size vessels to be used and cut costs dramatically.
One method to redress this problem is to use smaller size plugs in the tree. However, this solution restricts the choice of intervention methods, and some types of intervention will not be possible or the tubing must be pulled to gain access to the well. Therefore, the preferred choice is to use the full size plugs, to retain the freedom of choice.